The importance of providing protection against corrosion for steel articles used outdoors (such as fences, garbage cans, and automobile parts) is obvious, and coating the steel with zinc is a very effective and economical means for accomplishing this end. Zinc coatings are commonly applied by dipping or passing the article to be coated through a molten bath of the metal. This operation is termed "galvanizing," "hot galvanizing" or "hot-dip galvanizing" to distinguish it from zinc electroplating processes. The steel galvanizing process is very well-known in the art and, for example, is discussed in detail in The Making, Shaping, and Treating of Steel, United States Steel Corporation, 7.sup.th Edition, Pittsburgh, 1957, pages 660-673, and the 10.sup.th edition, Lankford et al. (eds.), Association of Iron and Steel Engineers, Pittsburgh, 1985, pages 1173-1189, incorporated herein by reference. Galvanization processes generally fall into one of two types: batch hot-dip galvanizing, which is the hot-dip galvanizing of pre-formed articles by passing them one by one and in close succession through the molten zinc, and (2) continuous (strip) hot-dip galvanizing, in which steel in coiled form from the rolling mills is uncoiled and passed continuously through the galvanizing equipment, continuity of operation being achieved by joining the trailing end of one coil to the leading end of the next.
Batch galvanizing is an old and well-known process, having been practiced for over 200 years. The basic steps in the batch galvanizing process include: alkaline or acid degreasing followed by pickling (usually in hydrochloric acid or sulfuric acid) to remove rust and clean the surface of the steel; fluxing to protect the active surface of the steel from oxidation and to improve the wetting of the steel surface by molten zinc in the galvanization step; and dipping the steel in a bath of molten zinc. Continuous galvanization is similar, except that fluxing is typically not included since there is generally no significant delay before the prepared steel is dipped in the molten zinc. Alternatively, in a continuous galvanization process, the steel may be placed in a furnace and subjected to a reducing atmosphere prior to dipping in the molten zinc. Batch galvanization and continuous galvanization have some other very significant differences:
(1) The steel article or sheet is dipped in the molten zinc for a much longer time in batch galvanization (three minutes, as compared with about ten seconds in a continuous process); PA0 (2) The batch process forms zinc iron alloys at the steel surface, while the continuous process generally does not; PA0 (3) Galvanized steel from a batch process generally cannot be deformed significantly, while the product from a continuous galvanization process generally can (requiring that batch galvanized items generally be formed prior to galvanization); PA0 (4) The thickness of the film formed in batch galvanization is about 75 .mu.m, while the film formed in the continuous galvanization process is only about 20 .mu.m; and PA0 (5) The steel sheets used in continuous galvanization are generally thinner than those used in batch galvanization. PA0 (1) While the operable time window between surface cleaning and the galvanization step is extended using the flux, that interval before oxidation begins is still relatively short (about two hours); PA0 (2) Dipping the fluxed article in the molten zinc bath produces hydrogen chloride and other toxic fumes; and PA0 (3) While it is desirable to include aluminum in the zinc bath in order to provide an anti-corrosion benefit to the galvanized coating, the chloride in conventional flux reacts with aluminum in the zinc bath, rendering the galvanization process ineffective. PA0 (1) The fluxed article is compatible with the use of aluminum in the molten zinc galvanizing bath; PA0 (2) A much longer delay (up to five, or even ten, days) is possible between the fluxing operation and the galvanization of the article; PA0 (3) No hydrogen chloride or other toxic fumes are formed when the article is dipped in the molten zinc galvanizing bath; PA0 (4) The fluxing process is inexpensive and provides good strong alloy coatings on the article; PA0 (5) The fluxing process is robust, operating effectively under a wide range of processing conditions; PA0 (6) The fluxing process appears to make galvanization less sensitive to the silicon content of the article being galvanized (i.e., the so-called Sandelin effect is minimized); and PA0 (7) The present invention allows the pickling step and the fluxing step to be combined into a single step thereby significantly simplifying the galvanization process. PA0 (a) degreasing said steel article by dipping it in an alkaline solution; PA0 (b) pickling said steel article by dipping it in an acid solution; and PA0 (c) fluxing said steel article by electroless plating on the surface of said steel article a layer of a metal, particularly those metals described above. PA0 (1) the toxic fumes which are formed when the fluxed steel is dipped in the molten zinc are eliminated; PA0 (2) a more uniform coating is formed in the galvanization process; and PA0 (3) the steel strip may be heated to a higher temperature prior to being galvanized, thereby minimizing temperature loss of the zinc bath.
Flux protects the steel surface from oxidation during any delay prior to the time the steel object is dipped in the molten zinc galvanizing tank. Flux is typically used in a batch galvanization process but not in a continuous process, since either there is little or no delay prior to the galvanization step in a continuous process or, alternatively, the sheet is deoxidized in a reducing atmosphere. Essentially one type of flux is currently used in industrial galvanization. In this conventional flux process, the steel sheet or object is dipped in an aqueous solution containing ammonium chloride and zinc chloride. This forms a zinc ammonium chloride film on the surface of the object or sheet. Even if the specific compounds used in the flux process are varied, they generally contain chloride salts. While this process does prevent oxidation of the steel surface, it also presents some significant problems:
The basic galvanization process, as well as a variety of attempts to address the problems associated with conventional fluxes, discussed above, are well-known in the art and exemplified by the following references.
The Making, Shaping and Treating of Steel, United States Steel Corporation, 7.sup.th Edition, 1957, Pittsburgh, Chapter 39, pages 660-673, and the 10.sup.th edition, Lankford et al. (eds.), Association of Iron and Steel Engineers, Pittsburgh, 1985, pages 1173-1189, contains a description of galvanization and the conventional processes used to galvanize steel.
Japanese Published Patent Application 07/233,459 (Toho AEN KK), published Sep. 5, 1995, describes a flux which comprises an aqueous solution of tin chloride and ammonium acetate. The flux is taught to be used prior to the galvanization of wires in a zinc-aluminum bath.
Japanese Published Patent Application 05/195,179 (Fuji Kogyo KK), published Aug. 3, 1993, describes a flux solution used for hot-dip zinc-aluminum galvanization, comprising an aqueous solution of MnCl.sub.2.4H.sub.2 O, zinc chloride, tin chloride and potassium formate.
Japanese Published Patent Application 05/148,602 (Fuji Kogyo KK), published Jun. 15, 1993, describes a flux solution used in a zinc-aluminum galvanizing process, comprising zinc chloride, tin chloride, potassium formate and hydrochloric acid in an aqueous solution.
Japanese Published Patent Application 05/117,835 (Sumitomo Metal Mining Co./Tanaka AEN Metsuki KK), published May 14, 1993, describes a flux, used in a hot-dip galvanizing process, comprising an aqueous solution of ammonium chloride, zinc chloride, bismuth chloride or stannous chloride, together with an alcohol.
Japanese Published Patent Application 04/157,146 (Sumitomo Metal Mining Co.), published May 29, 1992, describes a flux used for hot-dip zinc-aluminum galvanization, comprising zinc chloride, tin chloride, and the chloride of at least one alkaline metal element.
Although the two process are completely different, it should be noted for the sake of completeness that tin is known for use as a component of soldering flux, see, for example, U.S. Pat. No. 4,954,184 (Day Manufacturing Company, Inc.), issued Sep. 4, 1990.
British Patent 1,502,673 (BASF), issued Mar. 1, 1998, describes an aqueous flux, which gives off only low levels of fumes and smoke when used prior to hot-dip zinc galvanization, comprising zinc chloride, potassium chloride, and optionally components selected from sodium chloride, ammonium chloride and aluminum chloride.
U.S. Pat. No. 5,292,377, Izeki, et al. (Tanaka Galvanizing Co./Sumitomo Metal Mining Co.), issued Mar. 8, 1994, describes a process for galvanizing steel with a zinc/aluminum coating to enhance the corrosion resistance of the finished product. This process is an adaptation of the conventional fluxing process to try to make it compatible with the inclusion of aluminum in the zinc bath. Specifically, the flux comprises zinc chloride or stannous chloride, together with an alkaline metal or alkaline earth metal chloride and an alkyl quaternary ammonium salt or alkyl amine.
It should be noted that none of the references, discussed above, teaches the deposition of a metallic element, particularly tin, or a mixture of copper and tin, on a steel article for use as a flux prior to galvanization.
U.S. Pat. No. 4,505,958, Lieber et al. (Hermann Huster GmbH & Co.), issued Mar. 19, 1982, describes a process for hot dip galvanizing workpieces of steel or iron materials wherein treatment with a fluxing agent is omitted. After being cleaned, the workpieces are coated with a metal layer, comprising aluminum, lead, cadmium, copper, nickel, bismuth, zinc, tin, and alloys of these metals. This layer replaces the previously customarily applied fluxing agent layer and are the steel or iron materials are subsequently immersed into zinc melt with their surfaces in a dry state.
It should be noted the metal layer deposited onto the steel surface in Lieber et al. is thicker (i.e. about 100-120 nm) than that of the preferred embodiment of the present invention, which is from about 5 to about 50 nm. This tends to result in irregularities in the zinc coating formed in Lieber et. al. Additionally, Lieber et al. does not disclose the use of a mixture of copper and tin (i.e. a non-alloy) as a flux to coat the metal surface prior to galvanization.
U.S. Pat. No. 4,285,995, Gomersall (Inland Steel Co.), issued Aug. 25, 1981, describes a method of increasing the rate of formation of zinc-iron alloy when hot-dip galvanizing a ferrous metal strip to effect complete alloying of the hot-dip zinc coating on at least one side of the strip. In this method, at least one lateral surface of the ferrous strip is coated with metallic copper, which is then heated in a non-oxidizing atmosphere to a temperature sufficient to diffuse a portion of the copper coating into the ferrous metal strip and thereafter hot-dip galvanizing the strip.
It should be noted that Gomersall does not teach the use of a mixture of copper and tin to coat the steel surface prior to galvanization and Gomersall also requires heating the coated surface prior to galvanization wherein the present invention does not require this step.
It has now surprisingly been found that if a very thin metallic film, such as tin metal or, more preferably, a metallic film constituting a mixture of copper and tin, is deposited on a steel article (or sheet) as a flux, prior to galvanization, a number of significant benefits are realized:
The present invention, its variations and its many benefits are described in greater detail below.